Baseband line coding is a simple form of encoding a signal for communication that is conventionally undertaken in various processes and devices ranging from cellular telephones to computers. Baseband line coding, or simply “baseline” coding, yields several benefits. Those benefits include spectrum shaping and relocation without requiring filtering, simplified clock signal recovery, improved bandwidth usage and error detection capabilities.
In addition, baseband line coding allows DC components in the signal to be eliminated. Though this improves AC coupling, the elimination of DC component from the signal spectrum can cause baseline wander. Baseline wander shifts the position of the signal relative to thresholds, causing the magnitude of the signal to be misinterpreted and ultimately leading to severe erosion of fidelity and an unacceptably low signal to noise ratio.
Reading data from a hard disk drive is one example of baseline coding where baseline wander is routinely encountered. The problem becomes more acute as the lower part of the signal spectrum is used, such as when perpendicular magnetic recording is used. Typically, bits of information are stored on a magnetic medium of a hard disk drive. The bits are read as a voltage (or current) signal by a read/write head. The signal is amplified and sent to an analog to digital (A/D) converter, where it is converted from analog to digital form (a sequence of numbers, or bits). The digital bits are detected or remodulated and decoded into the information that was stored on the hard disk drive.
In the amplification of the low voltage representing the bits of information, high pass filters are employed which remove a portion of a signal spectrum around DC. This loss of the signal spectrum around DC leads to a degradation of the bit error rate (BER) when detecting the digital bits. To improve the BER, several methods have been designed to compensate for baseline wander and specifically compensate for the loss of the signal spectrum around DC. Decision feedback based schemes and error feedback based schemes are examples of two schemes used with a detector to compensate for the DC loss of signal energy and the resulting loss in BER.
These schemes, however, suffer from a delay in a feedback path of the detector. The feedback delay includes intrinsic latencies associated with the detector and the extrinsic latencies of computational delays such as an add of the feedback path. A portion of the delay in each of the schemes is typically caused by the detector having an implementation of a Viterbi algorithm in silicon. Many previous baseline wander compensation schemes, nevertheless, did not consider the delay associated with the detector. As discussed in Read Channel Issues in Perpendicular Magnetic Recording, Gopalaswamy, et al., IEEE Transactions on Magnetics, Vol. 37, No. 4, July 2001, at 1929 (incorporated herein by reference), the delay may degrade the above schemes until employing the schemes is worse than uncompensated detection of the signal. The Viterbi algorithm associated with the detector, however, assists in accurately and efficiently determining the digital bits received through the channel.
Accordingly, what is needed in the art is an improved system and method for remodulating data without being negatively effected by detector delay.